The present invention relates to high temperature superconductors, more particularly to the use thereof in filters suitable for electronic applications such as those involving radar or communications.
High temperature superconductor (abbreviated “HTS”) filters have already seen some commercial success in the capacity of operation in or with receiver antennae in cellular phone base stations. Nevertheless, the potential exists for expanded use of HTS filters. The main impediment to extension of HTS filters to emitter antennae applications is the observed “surface impedance nonlinearity” that is associated with the dependence of the HTS filters on the microwave field power level. This nonlinearity occurrence generates undesirable “intermodulation” products that are particularly detrimental to radar and secure communication applications. Recent work asserts the origin of this phenomenon as an intrinsic property of HTS, and hence concludes that this phenomenon cannot be eliminated.
At the front end of any microwave receiver-antenna is a filter that cuts off (“excises”) all frequencies outside a predetermined frequency window (“bandpass”) to prevent the totality of airways signals from overwhelming the device. The operational principle of such filters may be described in terms of a “resonator cavity,” according to which the cavity is designed to resonate at a predetermined frequency window (“bypass band”), where transmission is at its maximum, while at all other frequencies transmission is strongly suppressed. In practice, such a microwave cavity is realized by a series of inductively coupled narrow copper “strips” (“poles”) that can be arranged in a variety of configurations. For pertinent disclosure, see, e.g, Zhi-Yuan Shen, High-Temperature Superconducting Microwave Circuits, Artech House, Boston, May 1994, incorporated herein by reference, esp. Chapter Four. The terms “strip” and “pole” have been used interchangeably in the technical realm of HTS-based filters.
Shen at page 116 illustrates one such geometry (Shen, FIG. 4.14) and its corresponding bypass band (Shen, FIG. 4.15). More specifically, Shen states that his FIG. 4.14 shows “[a]four-pole superconductive microstrip filter layout. The substrate is LaAlO3.” Shen states that his FIG. 4.15 shows “[m]easured transmission response at 77K of a filter fabricated with a postannealed YbaCuO on 425-μm-thick substrate.” The box-like bypass band is roughly comprised of the sum of slightly shifted gaussian-shaped bypass bands from each of the poles. The more poles, the closer the bypass band shape is to an ideal “box” shape. On the other hand, increase in the number of poles is associated with increase in losses, which in turn degrades the bypass band sharpness (e.g., introduces “skirts”). The latter characteristics determine the degree to which signals outside the nominal bypass band are mixed in.
The same kind of unwanted mixing is also generated whenever the poles' surface impedance is nonlinear, i.e., dependent on the microwave signal power level. The magnitude of these undesirable admixtures, known as “intermodulation products,” is a key performance measure of a filter. For the common copper-based filters, in which copper is to a high degree a linear material, a popular approach to minimization of intermodulation products is to increase the number of poles. This approach constitutes a tradeoff between the bypass band sharpness on the one hand, and an increase in the device volume and its noise level on the other hand. The latter effects represent disadvantages that are particularly important for military and civilian applications in which size and bypass band sharpness are key. Examples of such applications are those involving antennae arrays in radar, as well those involving compact, sensitive antennae.
The discovery in 1986 of the HTS family of materials led to the suggestion of replacing the copper strips in microwave filters with HTS strips. The distinct advantages of HTS-based filters are their significantly lower losses (which are, depending on temperature, by about two to three orders of magnitude) and their relatively small size. Due to these and other advantages (such as affordability, reliability and low maintenance), as well as due to the advent of maintenance-free small-volume closed-cycle coolers, HTS-based filters have become a commercial reality. Notable among the companies involved in this line of business are Superconductor Technologies Inc. (STI) of Santa Barbara, Calif., and Conductus, Inc. of Sunnyvale, Calif. STI recently completed a merger with Conductus, and can be contacted at 460 Ward Drive, Santa Barbara, Calif., 93111. The current deployment of HTS-based filter units in over a thousand commercial wireless base-stations represents the first large scale application of such devices. See pertinent disclosure by B. Willemsen, “HTS Technology for Commercial Wireless Applications,” Journal of Superconductivity (in press, 2003), incorporated herein by reference. Moreover, the sharp bandpass and low losses characterizing HTS-based filters are expected to become increasingly influential in the increasingly crowded civilian cellular phones spectrum. HTS-based filters have military implications as well. For instance, the United States Navy is interested in extending the applicability of HTS-based filters to higher power emit antennae, such as may be used for radar.
The technological progress in HTS-based filter utilization is hampered, however, by the observed HTS surface impedance nonlinearity, i.e., its dependence on the microwave field power level. See pertinent disclosure by the following papers incorporated herein by reference: P. P. Nguyen, D. E. Oates, G. Dresselhaus, M. S. Dresselhaus and A. C. Anderson, Phys. Rev. B 51, 6686 (1995); Y. M. Habib, C. J. Lehner, D. E. Oates, L. R. Vale, R. H. Ono, G. Dresselhaus and M. S. Dresselhaus, Phys, Rev. B 57 13833 (1998). Surface impedance nonlinearity has been observed in low temperature superconductor (abbreviated “LTS”) films such as Niobium Nitrate (NbN). See pertinent disclosure by the following papers, incorporated herein by reference: P. P. Nguyen, D. E. Oates, G. Dresselhaus, M. S. Dresselhaus and A. C. Anderson, Phys. Rev. B 57, 6686 (1995); Y. M. Habib, C. J. Lehner, D. E. Oates, L. R. Vale, R. H. Ono, G. Dresselhaus and M. S. Dresselhaus, Phys, Rev. B 57, 13833 (1998). Surface impedance nonlinearity has also been observed in HTS films of YBCO and BSCCO and TBCCO (the Bi2Sr2CACuO and Tl2Ba2CaCuO groups). See pertinent disclosure by the following papers, incorporated herein by reference: J. H. Claasen, J. C. Booth, J. A. Beall, L. R. Vale, D. A. Rudman and R. H. Ono, Supercond. Sci. Technol. 12, 714 (1999); J. C. Booth, L. R. Vale, R. H. Ono and J. H. Claasen, Supercond. Sci. Technol. 12, 711 (1999); H. Claasen, J. C. Booth, J. A. Beall, D. A. Rudman, L. R. Vale and R. H. Ono, App. Phys. Lett. 74, 4023 (1999); J. C. Booth, J. A. Beall, D. A. Rudman, L. R. Vale and R. H. Ono, Journal App. Phys. 86, 1020 (1999). As of yet there is no consensus regarding the origin of surface impedance nonlinearity. See pertinent disclosure by D. E. Oates, M. H. Hein, P. J. Hirst, R. G. Humphreys, G. Koren and E. Polturak, “Nonlinear Microwave Surface Impedance of YBCO Films: Latest Results and Present Understanding,” Physica C (Superconductivity), volumes 372–376, part 1, pages 462–468 (1 Aug. 2002; available online 9 Apr. 2002), incorporated herein by reference.
Recent YBCO data provides clear evidence that nonlinearity is observed in high quality films and is enhanced by factors such as magnetic/non-magnetic impurity doping (Zn, Ni) and oxygen underdoping. See pertinent disclosure by the aforementioned D. E. Oates et al., Physica C 372–376, 462 (2002). It has been suggested that vortex motion in large-angle grain and twin boundaries, which are ubiquitous in HTS, are the root cause for the nonlinearity phenomenon. See pertinent disclosure by the following papers, incorporated herein by reference: M. Coffey and J. R. Clem, Phys. Rev. B 48, 342 (1993); M. Benkraouda and J. R. Clem, Phys. Rev. 53, 5716 (1996); J. McDonald, J. R. Clem and D. E. Oates, Phys. Rev. B 55, 11823 (1997); J. Halbritter, Journal of Superconductivity 10 91 (1997). This attribution to vortex motion is motivated by the observation that such defects act as Josephson junctions; since vortex motion in a Josephson junction is nonlinear, it is reasoned, the ensuing surface impedance is nonlinear as well. While this mechanism is certainly at work it has proved to be quantitatively unable to account for the observed nonlinearity at elevated power levels. See pertinent disclosure by the aforementioned D. E. Oates et al., Physica C 372–376, 462 (2002). See pertinent disclosure also by H. Xin, D. E. Oates, A. C. Anderson, R. L. Slattery, G. Dresselhaus and M. S. Dresselhaus, IEEE Trans. Microwave Theory and Techniques, 48, 1221 (2000), incorporated herein by reference.
A competing proposition is that the nonlinearity is intrinsic to the highly correlated electron state (the condensate state) that underlies the superconductivity phenomenon. This proposition (“intrinsic nonlinearity”) is consistent with the observed nonlinearities in LTS and HTS that are qualitatively different, and with the observed nonexistent frequency dependence of the nonlinearity effect. See pertinent disclose by the aforementioned D. E. Oates et al., Physica C 372–376, 462 (2002). This proposition was further corroborated by calculations that quantitatively explain recent data of high quality, optimally oxygenated HTS sample. See D. Agassi and D. E. Oates, “Nonlinear Surface Reactance of a Superconductor Strip,” Journal of Superconductivity, Vol. 16 No. 5, pp. 905–961 (October 2003).
Encased YBCO (YBa2Cu3O7-δ) strips (typically 300 nm thick, 100μ wide and on the order of several centimeters long) exhibit, at elevated power levels, an amount of nonlinearity sufficiently large to introduce intermodulation products to a degree that is deleterious for radar applications. If HTS emit-filters are to become a reality, then, nonlinear microwave response must be controlled and reduced. It is thus essential that the nonlinear surface impedance of HTS filters be reduced at microwave frequencies, in order to enhance the performance of HTS filters and extend their operational range to power applications such as those involving transmitting antennae and radar.
The following United States patent documents, incorporated herein by reference, pertain to superconductivity and superconductor devices, especially such involving high-temperature superconducting materials: Fuke et al. U.S. Pat. No. 6,529,092 B2 issued 4 Mar. 2003; Eden U.S. Pat. No. 6,516,208 B1 issued 4 Feb. 2003; Wikborg et al. U.S. Pat. No. 6,463,308 B1 issued 8 Oct. 2002; Talanov et al. U.S. Pat. No. 6,366,096 B1 issued 2 Apr. 2002; Hershtig U.S. Pat. No. 6,212,404 issued 3 Apr. 2001; Adam U.S. Pat. No. 6,094,588 issued 25 Jul. 2000; Mansour U.S. Pat. No. 6,041,245 issued 21 Mar. 2000; Larson et al. U.S. Patent Application Publication 2002/0130729 A1 published 19 Sep. 2002; Larson et al. U.S. Patent Application Publication 2002/0130716 A1 published 19 Sep. 2002.